The invention relates to a process for preparing 6,10- and 6,9-dimethyl-5,10-undecadienyl-2-ones by telomerizing isoprene with alkylacetoacetates using transition metal compounds as catalysts, followed by saponification and decarboxylation of the .beta.-ketoester obtained.
When using the asymmetric diene, isoprene, there are 12 possible telomeric products, taking into account the linkage pattern of isoprene units in the direction of attack of the nucleophile in the 1, or 3 position and the E,Z-isomerism of the internal double bond. This number is potentiated by using a nucleophile with several acid hydrogen atoms.
Of the 12 possible telomeric products, three have the desired head-to-tail linkage pattern analogous to that in naturally occurring products.
Hitherto, processes disclosed in the prior art have only led to products in which a very small percentage of this type of telomer, if any, can be detected. A process for the preparation of octadienyl hydrocarbon chains is known from J. Berger, J. fur prakt. Chem. 327 (1985) 634. Selectivities relating to head-to-tail linkages of &lt;8% are obtained using the route described there.
In G. Hata, J. Org. Chem. 1971 (15), 2116 and Zakharkin et 30 al., Zh. Org. Khim. 1988, (24) 2325, no reference is made to the production of compounds with the desired heads-to-tail linkage when using the reaction mentioned.
Compounds of this type, in particular .alpha.-geranylacetone, however, are of great importance as key components for the synthesis of vitamin E.